This invention relates generally to a technique and apparatus for measuring capacitance that is connected as part of a circuit of other passive elements and which is subject to having an interfering signal induced or generated therein. Although not so limited, the techniques of the present invention are particularly useful for measuring capacitance along a telephone communication line.
There are many occasions wherein the capacitance of a telephone line is desirable information. Such capacitance measurements are made regularly by telephone companies. One application is for determining the location along a lengthy telephone cable of an "open" or break in a wire of the cable. The normal capacitance value per unit length is known for telephone cables. The measurement of the capacitance serves to provide information as to the distance down the cable from the point where the measurement is being taken to where the open condition exists. Repair persons can then be sent to the proper location along the cable to effect the repair.
Another telephone application of capacitive measurement is for determination of the number of telephone sets that are connected to a telephone line. Each telephone set contains a capacitor as part of its ringing device.
Another application of measuring the capacitance of a telephone line is to determine whether an undesired branching line (commonly called a bridge tap) exists somewhere on the line. Yet another telephone application is to determine the capacitance associated with an impedance matching circuits to see if the capacitance value is correct or not.
In all of these telephone line capacitance measurement applications, the value of the capacitance to be measured is intermingled with resistances and inductances as part of the line itself or as part of devices attached to the line. In some cases of faulty lines being tested, there is additionally a finite shunt conductance across the telephone line. These conditions make it impossible to separate out the capacitive element in the telephone line by itself for easy measurement. Thus, the problem exists for measuring that capacitance accurately without the other resistive and inductive elements adversely affecting the results.
Furthermore, since telephone lines are often placed in close proximity to power lines, there may be a voltage induced therein having a frequency of that of the power line or some harmonic thereof. An accurate capacitance measurement technique must be able to operate without its results being affected by such induced voltage. These voltages are especially high when a capacitive measurement is made between one conductor of a telephone line and ground potential, a measurement often made in determining the location of a break of one conductor of a telephone line pair.
Common alternating current methods of measuring capacitance cannot be used in such an environment because the capacitance cannot be separated from resistances and inductances that are present. A telephone line is a selective network for alternating current signals with some frequencies being rejected and others accepted. Therefore, alternating current measurements of telephone lines give results that depend upon the parameters of the transmission media between the measuring point and the capacitance to be measured. The capacitance itself is not accurately measured.
It is thus necessary to use low frequency techniques that respond to a charge that the unknown capacitance can absorb. Most telephone central offices today use a crude technique wherein a large direct current voltage is applied to the telephone line through a current activated meter by the closing of a switch. The amount of "kick" of the pointer of the meter during charging gives the operator an indication of the amount of charge that has passed from the voltage source to the capacitance of the telephone line. This method is extremely inaccurate but it does overcome the problem of series inductance in the circuit along with the capacitance being measured.
An alternate technique is to charge the capacitance of a telephone line from the testing point by application of a direct current voltage thereto and then use precision measurement circuits to measure the stored charge within the unknown capacitance of the telephone line, from which that capacitance value can be calculated. There are many problems in adapting such a general technique to telephone line capacitance measurements, however, because of extraneous circuit elements and induced voltages. The undesirable effects of shunt and series resistances that exist as permanent attachments to the capacitance to be measured can be eliminated if the measuring device has a zero input impedance at low frequencies. But use of a zero input impedance approach is undesirable for another reason: input circuit protection of the measuring device must be provided. Since telephone cables typically extend into an electrical environment that is not well controlled, the possibility of sudden large voltages appearing on the telephone cable at some instance of time is very likely. It is possible to seriously damage a measuring instrument that does not have some current limiting element, such as a series resistance, at its input.
Therefore, it is desirable to have a capacitance measuring device that is not affected by the existance of series and shunt resistances but which can still employ a series impedance in the input of the device as current limiting protection against voltage surges. It is an object of this invention to provide such a technique and apparatus.
It is a further object of this invention to provide a technique and apparatus for measuring remote capacitance values that are insensitive to induced or generated interference alternating current signals.
It is another object of this invention to provide a technique and apparatus for measuring capacitance in a circuit that may have a steady state direct current voltage applied thereto.
It is yet another object of this invention to provide a technique and apparatus for measuring capacitance that is insensitive to induced random noise.